Air traffic control is a relatively new concept. Although airplanes were invented in 1903 by the Wright brothers, aviation did not become widespread until World War I. As traffic increased, a control system became necessary; at first, pilots used portable radio devices to communicate to people on the ground. After radar was invented during World War II, controllers started to use it to track both civilian and military aircraft. It was not until the late 1950's that a comprehensive air traffic control system was deployed nationwide.
Mode S is an air traffic control that relies upon data link technology that uses discretely addressed interrogations to place aircraft in a controlled airspace. Mode S equipment includes ground stations with sensors, and transponders aboard aircraft. The interrogations that ground sensors send planes include a request for identification information of the target plane. Once the identity of the aircraft is known, the ground sensor can send uniquely addressed messages known as extended squitters to which other aircraft will not respond. Replies contain information such as altitude and position reports.
The Mode S extended squitter is modeled on a form of communication that was familiar to the aviation community. In a disciplined conversation, the Mode S squitter comprises a message payload according to a very controlled regimen, a sender address, a recipient address and a parity check to assure that the message has not been garbled. The extended squitter then mimics the language of the control tower to some extent. Rather than “SeaTac approach, American three nine nine leaving two three zero, descending to two zero zero,” the Mode S extended squitter has the sender's identity, the recipient's identity, and a message. Just as the discipline of the pilots and controllers on a frequency causes them to ignore transmissions that do not name them, where the recipient address is neither all zeros (all call), nor the address of the intended recipient, the Mode S transponder simply ignores the transmission.
A TCAS equipped aircraft, independent of any ground inputs, performs surveillance of nearby aircraft by issuing interrogations at 1030 MHz. The transponders reply to TCAS on the 1090 MHz bandwidth. The replies are received and decoded by the surveillance portion of the TCAS software and the information is then provided to the collision avoidance algorithms.
Detection and tracking of another Mode-S equipped aircraft is dependent upon the ability of the TCAS receiver to hear the Mode-S extended squitter (automatic broadcast from the other aircraft's transponder) and the ability of the TCAS to reach the other aircraft with an interrogation. The designed parameters of the signal is especially useful as the power and the transmission radius of the messaging are small enough to assure that only relevant aircraft are involved in the conversation. The capability to receive another of transponder's extended squitter and interrogation replies is determined by the Minimum Triggering Level (MTL) of the TCAS RF receiver. The more sensitive the receiver, the lower the MTL.
Referring to FIG. 1, an exemplar 1 of a Mode-S extended squitter includes a control segment 2, 8 bits in length indicating the nature of the traffic; an address segment 3, 24 bits in length and configured, as indicated above, to uniquely designate the recipient and the recipient of the extended squitter or to place an “all call” to aircraft in the vicinity; a message payload segment 4 of exactly 56 bits in length; and a parity segment, indicative of the content of the message of 24 bits in length. By maintaining the discipline of message purpose, length, and format, a crowded airspace will still allow a great deal of interaction to assure safe passage through the airspace.
In military applications, relying on Mode-S extended squitter traffic for positioning and direction is useful but also exposes an Achilles heel to an enemy. For instance, where a an element 10, includes an element leader 15 and two formation members 12 and 18, extended squitter traffic through TCAS avionics 7, allow the element leader 15 to set the direction for the element 10, having formation members 12 and 18, “form up” on the element leader 15. When the element 10, joins a larger formation 5, the element leader 15 may “bind on” to the formation leader 24; “forming up” among additional element members 21 and 27.
In communication, as well, with commercial aircraft 9, the formation 5, by its formation leader 24 can safely avoid interference with commercial aircraft 9. Where aircraft are below a radar horizon, for instance flying within close proximity to terrain, the ability to avoid commercial aircraft 9 can be especially useful.
An enemy 6 might use the Mode-S extended squitter to hunt or sabotage the formation. As indicated, the anti-collision capability Mode-S use allows relies upon an interrogation and reply constituting a steady stream of extended squitter traffic. If either an enemy fakes or “spoofs” an interrogation or reply, a military aircraft might be steered or directed into a second aircraft or into terrain. Additionally, if the extended squitter traffic is subject to enemy 6 eavesdropping, there is no possibility of attacking without advance notice. Thus, it is very advantageous to have a method of communicating through a extended squitter without exposing its operation to the enemy.
There is, therefore, an unmet need in the art for a secure extended squitter system that is not subject to spoofing nor will betray the location of a military aircraft.